proposal: the possibility of integrating fsnotify to update index internal cache

## Background

In v0.13.0 release , we introduce the cache ticker to update the internal index . However , per #148 , if we deploy multiple replicas without external cache (redis cluster) , and upload a chart to replica A , the replica B's cache index will not recognize the chart , and then result in cache missing until the ticker reached . Even worser , the replica B will never update if user not open up the cache ticker option .

## Proposal

I read through the comments in #148 , most of our users deploy CM with the same backend storage . Thus , multiple replicas(index cache) to one storage backend storage path .

With [fsnotify](https://github.com/fsnotify/fsnotify) , it allow us to register a watcher to fs , so it seems that we can register a index file watcher and listen the file operation event , emit a cache action event when fs event reaches rather than emit the event in post http request handler.

With above setup , I think we can achieve the HA goal which community expects.

The cache ticker should be kept for those storage providers who do not support fs event .

## Behavior changes

* local fs (in chartmuseum/storage) will impl a new interface(wraps the old storage interface) to integrate with the new fsnotify mechanism.
* The backend storage 's (cache) event emitters in `pkg/server/multitenant/handlers.go` and `pkg/server/multitenant/api.go` will be removed if it impls the new interface , for those storage do not impl the new interface , the emitter will be kept (as the fallback case).